The glyceraldehyde-3-phosphate dehydrogenases (GAPDHs) of three lactic acid bacteria, Streptococcus faecalis, Pediococcus cerevisiae and Lactobacillus acidophilus have been purified to electrophoretic homogeneity and antisera have been prepared against the enzymes of the first two-mentioned species. Immuno-diffusion experiments have confirmed the broad relationships observed in the aldolase study, namely, that the streptococci, pediococci and lactobacilli form a continuous and comparatively close evolutionary cluster while the aerococci and propionibacteria are more distant relations. Quantitative immunological procedures have verified much of the ordering and many of the interspecific relationships among the three genera reported in the aldolase study. The ubiquitous nature of the GAPDH has enlarged the relatedness group to include the heterofermentative lactobacilli and genus Leuconostoc. Hence, the common ancestry of the large group of gram positive facultative and obligate anaerobes has been confirmed.